In a semiconductor device which has been used as a power device, silicon has been mainly used as the semiconductor material. Silicon carbide (SiC), which is a semiconductor with a wider band gap than silicon, has good physical properties, such as thermal conductivity that is three times more than silicon, a maximum electric field intensity that is ten times more than silicon, and an electron drift speed that is two times more than silicon. In recent years, various institutes have conducted active research on applications of a power device which has a high dielectric breakdown voltage and low loss and can operate at a high temperature. A vertical semiconductor device including a rear surface electrode which has a low-resistance ohmic electrode on the rear surface side has been mainly used as the structure of the power device. Various materials and structures have been used for the rear surface electrode. As one of the materials and structures, for example, a laminate of a titanium layer, a nickel layer, and a silver layer (for example, see the following Patent Document 1) or a laminate of a titanium layer, a nickel layer, and a gold layer (for example, see the following Patent Document 2) has been proposed.
In a vertical semiconductor device using SiC which is typified by a Schottky barrier diode, a method has been used which forms a nickel layer on a SiC substrate, heats the nickel layer to form a nickel silicide layer, and forms an ohmic contact between the SiC substrate and the nickel silicide layer (for example, see the following Patent Documents 1 and 2).
As a method of forming the ohmic electrode, a method has been proposed which forms a laminate of a plurality of metal (Ni, Ti, and Al) layers on a SiC substrate and heats the laminate at a temperature of 700° C. to 1100° C. In the method, ohmic characteristics are obtained at, most preferably, a temperature of 800° C. (for example, see the following Patent Document 3). In addition, a technique has been proposed which radiates a laser beam to the rear surface of a SiC substrate to form an ohmic electrode in a low-temperature process (for example, see the following Patent Document 4).